NCERT Science: Heredity and Evolution

The one-line takeaway

Heredity is the passing of traits from parents to offspring through genes carried on chromosomes (made of DNA). Gregor Mendel worked out the basic rules of inheritance, including dominant and recessive traits. Variation among offspring, acted on by natural selection over long periods, drives evolution, the gradual change of species over time.

Heredity and Mendel

• Heredity is the transmission of characters from one generation to the next. The unit of inheritance is the gene, a segment of DNA located on a chromosome inside the nucleus.
• Gregor Mendel, working with pea plants in the nineteenth century, is the "father of genetics." From his breeding experiments he established:
  • Traits exist in pairs controlled by pairs of factors (genes), one from each parent.
  • Dominant and recessive traits: when two different forms of a gene (alleles) are present, one (dominant) is expressed and the other (recessive) is masked. For example, in his peas, tall was dominant over short.
  • Law of segregation: the paired factors separate during the formation of gametes, so each gamete carries only one of the pair.
  • Law of independent assortment: genes for different traits are inherited independently of one another.
• A genotype is the genetic make-up; a phenotype is the observable trait. A 3:1 ratio of dominant to recessive appears in the second generation of a single-trait cross.

Genes, DNA and chromosomes

• Chromosomes are thread-like structures in the nucleus made of DNA and protein. Humans have 23 pairs (46 chromosomes) in each body cell, of which 22 pairs are autosomes and one pair is the sex chromosomes.
• DNA (deoxyribonucleic acid) carries the genetic code as a sequence of four bases; its double-helix structure was described by Watson and Crick. A gene is a stretch of DNA that codes for a particular protein and hence a trait.

Sex determination in humans

• Sex is determined by the sex chromosomes. Females are XX and males are XY.
• The egg always carries an X chromosome; the sperm carries either an X or a Y. It is the father's sperm that determines the sex of the child (an X-bearing sperm gives a girl, a Y-bearing sperm gives a boy). This fact is socially important and counters the unjust blaming of mothers for a child's sex.

Variation

• Variation is the difference among individuals of a species. Sexual reproduction generates variation because offspring inherit a mix of genes from both parents, plus occasional changes (mutations).
• Variation is the raw material of evolution; it allows some individuals to survive changing conditions that others cannot.

Evolution and natural selection

• Evolution is the gradual change in the inherited characteristics of a species over many generations. Charles Darwin proposed the theory of evolution by natural selection in On the Origin of Species (1859).
• Natural selection ("survival of the fittest"): within a varying population, individuals better suited to the environment survive and reproduce more, passing on their useful traits, so over time the population changes and new species can arise. Darwin's finches and the peppered moth are standard illustrations.
• Evidence for evolution includes homologous organs (same basic structure, different function, indicating common ancestry, such as the forelimbs of humans, bats and whales), analogous organs (different structure, same function, indicating different ancestry, such as the wings of birds and insects), and fossils (the preserved remains of past life that record the sequence of change).
• Humans (Homo sapiens) are understood to have evolved from earlier primate ancestors over millions of years.

Key terms to fix

• Gene, DNA, chromosome: the unit of heredity, the molecule that carries it, the structure that holds it; humans have 46 chromosomes (23 pairs).
• Dominant versus recessive traits; Mendel's laws of segregation and independent assortment.
• Sex determination: females XX, males XY; the father's sperm decides the child's sex.
• Variation and mutation: the raw material of evolution.
• Natural selection (Darwin, 1859): differential survival and reproduction; homologous and analogous organs and fossils as evidence.

CAPF angle

The scientific fact that the father determines a child's sex is a powerful tool against gender-based discrimination and the social evils of female foeticide and dowry, themes that overlap with the human-rights and women's-safety concerns the forces engage with. Genetics also underpins DNA fingerprinting, a key forensic and identification technology used in crime investigation and disaster-victim identification, areas where the forces and the police work together.

Authored practice